This invention relates to a method capable of producing flake particles directly upon the surface of a rotating member having discrete serrations in the peripheral edge thereof by projecting a stream of molten material upon the moving surface, forming flake particles on the serrations.
As used herein the terms flake particles, flakes and flake refer to particles of relatively small size in the hundredths of an inch range, and the terms include particles frequently referred to as powders because of their small size.
A disclosure of valuable characteristics of flake particles and powders will be found in pending U.S. Patent Application Ser. No. 826,798, now Pat. No. 4,154,284 issued May 15, 1979, made by the same applicant as this application and assigned to the same assignee. A discussion of size and other properties that are desirable in particles and powders is found in a paper published in Solidification Technology, pp. 317-336, NCIC in 1974.
A large number of methods exist for the manufacture of metal flakes or powders. These range from various mechanical methods such as grinding or filing to casting methods utilizing water or jets to break up a molten metal stream.
A method of producing flake particles directly from a supply of molten material by the use of a rotating member having discrete serrations in the peripheral edge thereof in which the leading surfaces of the serrations contact the molten material and have formed thereon flake powders is disclosed in the patent application by the same inventor recited above.
Most of the prior art methods that are used to make flake particles have various deficiencies. For instance, an early method was to grind or chip or cut the ends of a wire or rod in progressive sectional slices, each ending up as a flake or powder particle. In this process, the wire rod had to be formed and mechanically worked to its appropriate cross section before the slicing operation, which was time consuming and an extra expense. The slicing, grinding, or chipping required multiple tool faces which wore and became dull, requiring replacement, sharpening, and other expensive treatments.
Other prior art methods using atomization and spraying are relatively uncontrolled from a particle size distribution and configuration standpoint. In these methods, an orifice is required to atomize and spray the molten material.
After the molten material is projected in a stream through the orifice it is contacted with a blast of air or other gas which breaks the stream up into small particles. The particles either cool and solidify in the air or gas stream, or are caused to strike a cool surface to accomplish this result. The particles resulting from these processes are of random shapes, size, and grain size.
In the spray atomization process, because some particles are very small, they may have had very rapid quench rates, and have very small grain size. However, this group remains a small portion of the total.
Another prior art method in the production of flakes is the process of ball milling particles produced by other methods such as those described above. The ball milling process flattens out spherically shaped particles by crushing the particles between the rolling balls.
Although it is known in the laboratory how to produce rapid quench rates as high as 10.sup.7 -10.sup.9 C. degrees per second and to produce grain sizes of less than 0.01 micron, from a practical commercial application standpoint, methods capable of producing the formed product are still being sought and have not been reported at quench rates uniformly above about 10.sup.4 C. degrees per second, except for the method disclosed in applicant's application recited above.
In the present process the formation of the materials into final flake particle form is carried out while the material is changing directly from the molten state, and therefore inorganic compounds having properties in the molten state similar to that of molten metals and metal alloys may be formed in substantially the same manner. The properties that must be similar to those of molten metal are the viscosity and surface tension in the molten state, as well as the compound having a substantially discrete melting point, rather than the broad continuous range of viscosities characteristic of molten glasses.
Materials conforming to the class for this invention and having such properties will have a viscosity in the molten state when at a temperature of within 25% of their equilibrium melting point in degrees Kelvin in the range of 10.sup.-3 to one poise as well as having surface tension values in that same temperature range in the order of 10 to 2500 dynes per centimeter.
The prior art discloses atomization of molten stream materials sprayed from an orifice upon the surface of a rotating copper roll. When the atomized stream strikes and splotches against the cool surfaces of the roll, rapid quenching takes place and a multiple series of random shaped flakes are formed. The present invention controls the shape and size of the final flake product. Controlling the shape and size, including thickness, are important in determining the physical properties of the product when the size of the product is very small.
Other prior art methods of handling molten materials to achieve rapid quenching of small particles thereof are discussed in U.S. Pat. No. 2,825,108 and U.S. Pat. No. 3,710,842.